Fluid bed gasifiers for coal and other carboneous materials operate by having an oxygen containing gas pass through a grid into a bed of fluidized solids and the gas reacts with the carbon containing solids to generate syngas. A widely used system is the Synthesis Energy Systems fluidized bed gasifier (“SES gasifier”), described in e.g. U.S. patent application Ser. No. 13/532,769. The basic structure of an SES gasifier comprises a vessel housing a headspace above a fluidized bed of the solid materials being gasified, and below the bed a conical grid through which the gasifying medium is introduced at sufficient velocity to fluidize the solid materials in the gasifier. Carbonous feed stock, usually as small particles, is delivered just above the grid. Steam and oxidant (either air or oxygen) are delivered from under the grid to fluidize and partially oxidize the feed stock Immediately underneath the grid and above other structures which are usually one or more additional vessels for ash cooling and processing etc., is an empty “plenum” space.
These gasifiers operate at temperatures from 900° C. to 1200° C. To protect the mechanical integrity of the grid, gas flow through the grid must be constantly maintained as long as the bed solids remain above the temperature at which the metal in the grid begins to lose a significant amount of strength. This temperature can be anywhere from 600 to 800° C., depending on the grid materials. For SES U-gas technology the grid temperature typically needs to be maintained below 700-800° C. If this gas flow is lost, for example due to power failure, the hot bed material will settle on the grid and the grid may be destroyed or severely damaged.
In addition to simple power failure, other equipment failure may also cause the loss of the gasifying gas flow. For example, steam is the most commonly used gasifying gas flow sent through the grid, and the steam system may fail leading to a loss of steam pressure, causing devastating damage to the grid.
It is therefore highly desirable to maintain a cooling gas flow through the grid under conditions where the fluidizing gas flow is lost.
The fluidizing gas must be initially available at slightly above the gasifier operating pressure which can be as high as 6 bars. Yet maintaining such a cooling gas flow is challenging because of the volume of gasses required and the pressure required. Additionally, fluidizing gas must be available even with complete loss of electrical power. Under those conditions, it would not be practical to store a gas in sufficient volumes under sufficient pressure for use to cool the bed solids to a safe point. The use of an easily vaporizable liquid (e.g. liquefied nitrogen) is equally not practical. Both would be prohibitively expensive and complex, not the least because of the need for (1) equipment to pump the liquid through a vaporizer, and (2) emergency power generation for the pump, or fuel-operated direct drive pumps.
There is the further complication that the cooling and fluidization must be done in such a way as to not damage the grid material or the refractory lining usually used in the chamber under the grid. For example, simply spraying excess water onto the underside of the grid using diesel driven pumps is not permitted due to the thermal shock to the grid and water damage to the refractory lining of the chamber under the grid.
Therefore, there is a need for a solution to the above problem when fluidizing gas flow is lost.